Many products are fabricated from polymeric materials by extrusion of the polymeric material while the material is in the heat-plastified state. Typical of such procedures is the melt extrusion of pipe, electrical conduit, and the like, and the extrusion of parisons in blow molding operations. Though the size of the product being formed is frequently such that a single extrusion die handles the total output of the extruder, it has long been recognized that, for some products, one extruder can supply the heat-plastified material at a rate adequate for two, or even more, dies. Thus, for example, an extruder can be equipped with two pipe extrusion dies, one for 2 in. dia. pipe, the other for 11/2 in. dia. pipe, and pipe of the two sizes can be extruded simultaneously.
Such simultaneous extrusion presents the problem of accomplishing the appropriate flow rate of polymeric material through each die. If the apparatus were to be used only to produce, e.g., two specified products, such as 2 in. and 11/2 in. polyvinyl chloride pipe, only two specific dies would be used and it would be possible to so design and construct the apparatus that operation of the extruder would always supply each die with plastified polymeric material at the rate appropriate for that die. In actual practice, however, it is necessary to achieve greater flexibility of production, and it is desirable to be able to use the extruder to produce a wider variation of products. Thus, for example, an extruder may be used to produce a 2 in. pipe and a 11/2 in. pipe simultaneously over one period of time, then to produce two 2 in. pipes simultaneously, and then to simultaneously produce pipes of entirely different diameters. To provide such flexibility of operation, and to achieve precise control of the flow to each die, prior-art workers have found it necessary to equip each die with an adjustable flow control valve. Such arrangements are disclosed, for example, in U.S. Pat. Nos. 2,653,352, 2,740,989, 2,952,873, 2,980,955, 3,046,602 and 3,193,878. See also Processing of Thermoplastic Materials, Ernest C. Bernhardt, 1959, Reinhold Publishing Corp., New York, New York, pages 246-248.
While apparatus employing multiple dies each equipped with an adjustable flow control valve have achieved considerable acceptance in the trade, use of a valve under the circumstances of melt extrusion, for example, is not a completely satisfactory procedure, and there has been a continuing need for improvement in apparatus of this general type.